The present invention relates to an improvement in a laser machining apparatus for finely machining a semiconductor circuit or similar workpiece by focusing a laser beam thereonto.
A laser machining apparatus of the type described generally focuses a laser beam to a machining position by either one of two different laser positioning systems. One of the positioning systems uses a mirror which is rotatably driven by a galvanometer and steers an incident laser beam, and an f.theta. lens which focuses the steered laser beam onto a workpiece. The other system shifts an XY stage or table loaded with a workpiece while maintaining a laser beam fixed in position, until a predetermined machining position coincides with the focusing position of the laser beam, as disclosed in U.S. Pat. No. 4,543,464.
The beam steering scheme mentioned above features an inherently high beam scanning rate. However, it has a drawback that the laser beam cannot be positioned with accuracy relative to a workpiece. Another drawback is that the beam spot cannot be reduced in radius while keeping the same scanning area because the focal length of the f.theta. lens has to be reduced for the decreased beam spot. While the stationary beam scheme is successful in determining an accurate machining position, it results in a bulky and expensive apparatus construction due to the need for a precision XY stage which is movable over a substantial distance and slow in movement.
A beam positioning system which constitutes an improvement over the above-discussed two systems is disclosed in U.S. Pat. No. 4,532,402. Specifically, the improvement uses an XY stage which is made up of an X stage and a Y stage and loaded with a workpiece. A galvanometer and a focusing lens are rigidly mounted on one of the X and Y stages for steering a laser beam. The XY stage roughly adjusts the position of the laser beam over a broad area, while a mirror mounted on the galvanometer steers the laser beam at a high speed over a small area around a desired machining position. This kind of approach, however, requires the XY stage to move over a broad area and requires a substantial period of time for moving the XY stage, while increasing the overall dimensions of the assembly. Since the final positioning of the laser beam is effected by the galvanometer, the accuracy of machining position is not satisfactory. Further, complicated control is needed for the XY stage and galvanometer.